The long-term stability of foams is an essential requirement in a wide number of applications ranging from food and cosmetics to biomedical implants and engineering low-weight structures. Foams are extensively used as an end product in food and cosmetics, where the long-term stability is essential to keep desired physical and chemical properties such as texture and rheological behavior [1, 2]. Well-established and emerging applications that use foams as an intermediate structure to produce macroporous materials are also widely spread in the engineering field to fabricate thermal insulating materials and low-weight structures [3-5], as well as in medicine to produce artificial implants and scaffolds for drug delivery and tissue engineering [6, 7]. As an intermediate material, the foam has to be stable enough to allow for the fabrication of structures with tailored porosity and pore size distribution.
However, foams are inherently thermodynamic unstable systems which tend to undergo rapid coalescence and disproportionation of bubbles, due to the markedly high interfacial energy associated with the gas-liquid interface.
The state-of-the-art method to inhibit the coalescence and disproportionation of bubbles in a foam is to use biomolecules (e.g. proteins and lipids) or long-chain surfactants (e.g. soaps and detergents), which adsorb at the gas-liquid interface reducing the foam overall free energy. However, since the adsorption of these molecules at the interface is most often a reversible process, no long-term stability can be achieved by this means. A practical solution to this problem has been the use of gelling agents to set the foam structure before coalescence and disproportionation takes place. This has also been accomplished by solidifying the foam liquid media (lamellas). Most of such setting processes are triggered by temperature changes, which limit the fixing mechanism to relatively thinned cross-sections where no significant temperature gradients are developed. Alternative setting mechanisms based solely on chemical reactions at the foam liquid media are also possible, but are either very specific for a given foam system and contain often toxic reactants.
Thus, there exists still a need for foams with improved long-time stability as well as means suitable to achieve such long-lasting foams.
In addition to surface active molecules, it was only recently recognized that partially-hydrophobic particles can also stabilize air bubbles in surfactant-free diluted suspensions [8-14]. Similarly to surfactant molecules, the adsorption of colloidal particles onto a gas bubble surface lowers the overall free energy of the gas-liquid interface. The reduction of the total free energy upon particle adsorption is achieved by replacing part of the gas-liquid interfacial area with solids, rather than reducing the interface tension as in the case of surfactant molecules [8, 9, 14]. The wetting properties of the adsorbing particle determine its position at the interface and therefore the amount of total gas-liquid interfacial area replaced. Particles exhibiting intermediate hydrophobicity (contact angle θ close to 90°) can replace a large area of the gas-liquid interface and thus are the most efficient in reducing the overall interfacial free energy. However, the interfacial adsorption of submicron-sized particles displaying contact angles as low as 20° can already reduce the interface free energy by more than a few hundred kTs, implying that particles are irreversibly adsorbed at the air-water interface even for slightly lyophobized particle surfaces [14].
The stabilization of air bubbles with partially lyophobic particles alone has been so far restricted to model experiments and to a few observations of single air bubbles on thin top layers in diluted suspensions [8, 9, 14].